Set min_sdk_version to be part of mainline modules

Modules contributing mainline modules (APK/APEX) should set
min_sdk_version as well as apex_available.

For now setting min_sdk_version doesn't change build outputs.
But build-time checks will be added soon.

Bug: 152655956
Test: m
Change-Id: If4a0beed9150f6bed96699e6a78f2a12f25cb78a
diff --git a/Android.bp b/Android.bp
index d560fdf..bdf7f8d 100644
--- a/Android.bp
+++ b/Android.bp
@@ -68,4 +68,5 @@
             enabled: false,
         },
     },
+    min_sdk_version: "29",
 }